1 files changed, 20 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 768616f..89fd3de0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -498,6 +498,26 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
   }
 
+  // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
+  // conversion.
+  // It works because bits coming from sign extension have the same value as
+  // sign bit of the original value; performing ashr instead of lshr
+  // generates bits of the same value as the sign bit.
+  if (Src->hasOneUse() &&
+      match(Src, m_LShr(m_SExt(m_Value(A)), m_ConstantInt(Cst))) &&
+      cast<Instruction>(Src)->getOperand(0)->hasOneUse()) {
+    const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+    // This optimization can be only performed when zero bits generated by
+    // the original lshr aren't pulled into the value after truncation, so we
+    // can only shift by values smaller then the size of destination type (in
+    // bits).
+    if (Cst->getValue().ult(ASize)) {
+      Value *Shift = Builder->CreateAShr(A, Cst->getZExtValue());
+      Shift->takeName(Src);
+      return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+    }
+  }
+
   // Transform "trunc (and X, cst)" -> "and (trunc X), cst" so long as the dest
   // type isn't non-native.
   if (Src->hasOneUse() && isa<IntegerType>(Src->getType()) &&